Apparatus and Method for Steerable Drilling

ABSTRACT

A method for drilling a wellbore comprises extending a rotatable drill string in the wellbore, where the drill string has a bottom hole assembly coupled to a bottom end thereof. A lower section of the bottom hole assembly comprising a steering apparatus is coupled to an upper section of the bottom hole assembly with a controllably adjustable clutch. A steering apparatus toolface angle is detected. The clutch is controllably adjusted to maintain the steering apparatus toolface angle within a predetermined range about a target steering apparatus toolface angle while rotating the upper section with the drill string.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to the field of drillingsystems, and more particularly to steerable drilling systems.

In directional drilling, for example horizontal drilling and geosteeringapplications it may be advantageous to use rotary steerable systems toprevent pipe sticking in the horizontal section. It may also bedesirable to have the ability to have a drilling motor and bent sub forchanging direction. In operation, the motor, and the bent sub may benon-rotating with respect to the borehole while changing direction. Atthe same time, it may be advantageous to have the drill string rotatingto prevent differential sticking and to reduce friction with theborehole wall. A system providing these features may provide improvedhole quality and drilling efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of example embodiments are considered inconjunction with the following drawings, wherein:

FIG. 1 shows a schematic example of a drilling system according to oneembodiment;

FIG. 2 is a schematic of an example bottom hole assembly containing anorienter having a clutch therein;

FIG. 3 is a diagram of an orienter having a clutch therein;

FIGS. 4A and 4B are views of an example steering apparatus in the bottomhole assembly;

FIG. 5 is a schematic of another example of a bottom hole assemblycontaining a clutched orienter;

FIG. 6 is a logic diagram of one method of using a clutched orienter foruse while rotating the drill string and steering;

FIG. 7 is a view showing an example of an acceptable range for asteering apparatus toolface about the target toolface;

FIG. 8 is a view showing an example of a hydraulic schematic forcontrolling a clutch;

FIG. 9 is a schematic of an example bottom hole assembly containing anorienter having a clutch therein;

FIG. 10 is a diagram of an orienter having a clutch therein; and

FIG. 11 is a schematic of another example bottom hole assemblycontaining an orienter having a clutch therein.

FIG. 12A is a schematic of an example bottom hole assembly containing anorienter having a clutch therein and an adjustable steering apparatus;

FIG. 12B is a schematic of one example of a controllable member of anadjustable steering apparatus; and

FIGS. 13A-13D show an example of an adjustable steering apparatus havinga controllably bendable drilling shaft.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thescope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION

Described below are several illustrative embodiments of the presentinvention. They are meant as examples and not as limitations on theclaims that follow.

As used herein, the term clutch is intended to mean a coupling mechanismfor transmitting torque between two relatively rotatable members. Thetorque transmission mechanism may provide for locked rotation betweenthe two relatively rotatable members. In addition, the torquetransmission mechanism may be variable such that there is a controlledslip between the two relatively rotatable members. Clutch examplesinclude, but are not limited to, a mechanical clutch, an electromagneticclutch, and a hydraulic clutch.

FIG. 1 shows a schematic diagram of a drilling system 110 having adownhole assembly according to one embodiment of present invention. Asshown, the system 110 includes a conventional derrick 111 erected on aderrick floor 112 which supports a rotary table 114 that is rotated by aprime mover (not shown) at a desired rotational speed. A drill string120 comprising a drill pipe section 122 extends downward from rotarytable 114 into a directional borehole, also called a wellbore, 126.Borehole 126 may travel in a two-dimensional and/or three-dimensionalpath. A drill bit 150 is attached to the downhole end of drill string120 and disintegrates the geological formation 123 when drill bit 150 isrotated. The drill string 120 is coupled to a drawworks 130 via a kellyjoint 121, swivel 128 and line 129 through a system of pulleys (notshown). During the drilling operations, drawworks 130 may be operated toraise and lower drill string 120 to control the weight on bit 150 andthe rate of penetration of drill string 120 into borehole 126. Theoperation of drawworks 130 is well known in the art and is thus notdescribed in detail herein.

During drilling operations a suitable drilling fluid (also called “mud”)131 from a mud pit 132 is circulated under pressure through drill string120 by a mud pump 134. Drilling fluid 131 passes from mud pump 134 intodrill string 120 via fluid line 138 and kelly joint 121. Drilling fluid131 is discharged at the borehole bottom 151 through an opening in drillbit 150. Drilling fluid 131 circulates uphole through the annular space127 between drill string 120 and borehole 126 and is discharged into mudpit 132 via a return line 135. A variety of sensors (not shown) may beappropriately deployed on the surface according to known methods in theart to provide information about various drilling-related parameters,such as fluid flow rate, weight on bit, hook load, etc.

In one example, a surface control unit 140 may receive signals fromdownhole sensors (discussed below) via a telemetry system and processessuch signals according to programmed instructions provided to surfacecontrol unit 140. Surface control unit 140 may display desired drillingparameters and other information on a display/monitor 142 which may beused by an operator to control the drilling operations. Surface controlunit 140 may contain a computer, memory for storing data, a datarecorder, and other peripherals. Surface control unit 140 may alsoinclude drilling models and may process data according to programmedinstructions, and respond to user commands entered through a suitableinput device, such as a keyboard (not shown).

In one example embodiment of the present invention, a steerable drillingbottom hole assembly (BHA) 159 is attached to drill string 120, and maycomprise a measurement while drilling (MWD) assembly 158, an orienter190, a drilling motor 180, a steering apparatus 160, and drill bit 150.MWD assembly 158 may comprise a sensor section 164 and a telemetrytransmitter 133. Sensor section 164 may comprise various sensors toprovide information about the formation 123 and downhole drillingparameters.

MWD sensors in sensor section 164 may comprise a device to measure theformation resistivity, a gamma ray device for measuring the formationgamma ray intensity, directional sensors, for example inclinometers andmagnetometers, to determine the inclination, azimuth, and high side ofat least a portion of BHA 159, and pressure sensors for measuringdrilling fluid pressure downhole. The above-noted devices may transmitdata to a telemetry transmitter 133, which in turn transmits the datauphole to the surface control unit 140. In one embodiment a mud pulsetelemetry technique may be used to generate encoded pressure pulses,also called pressure signals, that communicate data from downholesensors and devices to the surface during drilling and/or loggingoperations. A transducer 143 may be placed in the mud supply line 138 todetect the encoded pressure signals responsive to the data transmittedby the downhole transmitter 133. Transducer 143 generates electricalsignals in response to the mud pressure variations and transmits suchsignals to surface control unit 140. Alternatively, other telemetrytechniques such as electromagnetic and/or acoustic techniques or anyother suitable technique known in the art may be utilized for thepurposes of this invention. In one embodiment, drill pipe sections 122may comprise hard-wired drill pipe which may be used to communicatebetween the surface and downhole devices. Hard wired drill pipe maycomprise segmented wired drill pipe sections with mating communicationand/or power couplers in the tool joint area. Such hard-wired drill pipesections are commercially available and will not be described here inmore detail. In one example, combinations of the techniques describedmay be used. In one embodiment, a surface transmitter/receiver 180communicates with downhole tools using any of the transmissiontechniques described, for example a mud pulse telemetry technique. Thismay enable two-way communication between surface control unit 140 andthe downhole tools described below.

FIG. 2 shows an expanded view of one example of BHA 159. As showntherein, orienter 190 comprises a housing 210 that rotates with drillstring 120. Orienter 190 also comprises a shaft 170 that may becontrollably rotated relative to housing 210, as described below. Therelative motion of orienter shaft 170 relative to orienter housing 210allows BHA 159 to be considered to have an upper BHA section 156 thatmay rotate with drill string 120, and a lower BHA section 157 thatrotates at the same speed as the drill string, at a different speed asthe drill string 120, or that is substantially non-rotating. In oneexample, if drill string 120 is rotating slowly, orienter shaft 170 mayrotate in an opposite direction relative to drill string 120.

A drilling motor 180 may be attached to orienter shaft 170. In oneexample, drilling motor 180 may be a fluid powered positive displacementmotor using the Moineau principle known in the art. As fluid passesthrough drilling motor 180 it forces the motor shaft 175 to rotaterelative to motor housing 181. In one embodiment, the rotating motorshaft 175 passes through steering apparatus 160 and is coupled to, androtates bit 150. In the embodiment shown in FIG. 2, steering apparatus160 comprises a bent sub 169. The rotation of motor shaft 175 causes areaction torque on housing 181 and causes housing 181 to rotate in thereverse direction relative to the motor shaft 175 rotation. This reverserotation may also cause orienter shaft 170 to rotate relative toorienter housing 210 when orienter shaft 170 is not locked rotationallywith orienter housing 210 by a clutch.

Also referring to FIG. 3, in one embodiment, the relative reactiverotation of orienter shaft 170 relative to orienter housing 210 may beused to generate hydraulic power. In the example shown in FIG. 3, anangled swash plate 215 is coupled to shaft 170. As shaft 170 rotates, itforce pistons 220 to reciprocate in bores 221 in pump housing 222 thatmay be coupled to orienter housing 210. Solenoid valve 230 may beactuated by controller 240 such that hydraulic fluid is pressurized toforce piston 225 to move axially to force the clutch plates 226 and 227of clutch 228 into controllable contact. Alternatively, any suitablepositive displacement type hydraulic pump may be used. As shown, housingclutch plates 226 are engaged with orienter housing 210, for example byan axially extending spline arrangement (not shown) known in the art.Mating shaft clutch plates 227 may be engaged with orienter shaft 170using a similar spline arrangement. The splines allow axial movement ofthe clutch plates and also transmit torque when the housing clutchplates 226 are frictionally engaged with the shaft clutch plates 227.The amount of torque transmitted may be adjusted by varying the axialforce, and hence the friction, between the plates. In one example, thefrictional force may be adjusted such that there is controlled slippagebetween clutch plates 226 and 227.

FIG. 8 depicts one example of a hydraulic circuit 712 for operatingclutch 228 in the orienter 190. In this example, the frictional force onclutch 228 is adjusted by adjusting the fluid pressure acting on piston225. The fluid pressure is adjusted by controlling the flow resistancedownstream of piston 225. As shown in FIG. 8, hydraulic loop 712comprises a first flow restrictor 730 positioned in loop 712 on anupstream side of clutch 228 and a second flow restrictor 732 positionedon a downstream side of clutch 228.

Referring to FIG. 8, hydraulic circuit 712 may also comprise a firstvalve 734 positioned on the upstream side of piston 225 and a secondvalve 736 positioned in loop 712 on the downstream side of piston 225.The valves (734, 736) may each be actuated between an open position anda closed position in which the loop 712 is blocked between the firstvalve 734 and the second valve 736 in order to maintain the engagementforce between the clutch plates 226 and 227 of clutch 228. The valves(734, 736) may be actuatable by orienter controller 240.

Referring to FIG. 8, the loop 712 may comprise a pressure relief bypassline 738 for bypassing the first valve 734 and the second valve 736 whenthe fluid pressure in loop 712 exceeds a preset bypass pressure.Pressure relief bypass line 738 leads to the low pressure reservoir 740which provides the pumping fluid 714 to the pump 230. Dump valve 742releases an amount of the pumping fluid 714 from the loop 712 when thefluid pressure in the loop 712 exceeds a preset dump pressure.Accumulator 744 is in fluid communication with loop 712, and suppliesadditional pumping fluid to the loop when the fluid pressure in the loop712 is below a preset accumulator threshold pressure.

The pumping fluid 714 is drawn from reservoir 740 and pumped by the pump230 via a reservoir supply line 708 as the orienter shaft 170 rotatesrelative to the orienter housing 210 (see FIG. 3). Pumping fluid 714passes through check valve 710 to a manifold 713. Two lines extend frommanifold 713. A first manifold line 715 extends between the manifold 713and first valve 734. First flow restrictor 730 is positioned withinfirst manifold line 715 in order to control the flow rate of the pumped.A second manifold line 716 extends between the manifold 713 and apressure relief bypass valve 718 through line 738 to low pressurereservoir 740.

In operation, if the first valve 734 is closed, the fluid pressure inthe manifold 713 will increase as pump 230 pumps the pumping fluid 714until the fluid pressure exceeds the bypass pressure, at which point thepumping fluid 714 will pass through the pressure relief bypass valve 718to the reservoir 740. If the first valve 734 is open, the pumping fluid714 passes from the second manifold line 715 to a clutch actuation line720 which extends between the first valve 734 and the second valve 736.A clutch pressure line 722 extends between the clutch actuation line 720and a piston 225 so that the fluid pressure in the clutch pressure line722 is equal to the fluid pressure in the clutch actuation line 720.

Orienter 190 may be actuated to allow rotation of shaft 170 relative toorienter housing 210 by providing a fluid pressure in the clutchpressure line 722 which is less than a locking pressure which isrequired to provide a locked engagement force between clutch plates(226, 227). Such a fluid pressure may be achieved by selectivelyactuating valves (734, 736). As one example, first valve 734 may beactuated to the closed position while second valve 736 is actuated tothe open position. As a second example, both valves (734, 736) may beactuated to the closed position while the fluid pressure in the clutchpressure line 722 is less than the locking pressure. As a third example,both valves (734, 736) may be actuated to the open position if thepumping resistance in the loop 712 provides a fluid pressure in theclutch pressure line 722 which is less than the locking pressure. In oneexample, the pumping resistance in loop 712 may be adjusted by pulsingvalve 736 to maintain pressure in the clutch pressure line 722. Pressuresensor 726 may be monitored to provide a feedback input to controller240 (see FIG. 3) to allow a substantially constant pressure to bemaintained in clutch pressure line. The magnitude of the pressure may beadjusted by varying the frequency and duration of the pulses of valve736.

Orienter 190 may be actuated to prevent rotation of orienter shaft 170relative to orienter housing 210 by providing a fluid pressure in theclutch pressure line 722 which is greater than or equal to a lockingpressure which is required to provide a locking engagement force betweenthe clutch plates (226, 227) (see FIG. 3). As one example, first valve734 may be actuated to the open position while second valve 736 isactuated to the closed position, thereby causing the fluid pressure inthe clutch pressure line 722 to increase to the locking pressure (whichlocking pressure is less than or equal to the bypass pressure asdetermined by the pressure relief bypass line 738. First valve 734 maythen be closed in order to “trap” the locking pressure in the clutchpressure line 722. Unlocking of orienter 190 may be achieved byactuating second valve 736 to the open position in order to permit thepumping fluid 714 to move from the clutch pressure line 722 back to thereservoir 740.

In one embodiment, also referring to FIGS. 4A and 4B, bent sub 169comprises an upper section 161 and a lower section 162. In one example,the upper section 161 and the lower section 162 are adjustable such thatthe longitudinal axis 163 of upper section 161 may be adjustablyangularly offset with respect to the longitudinal axis 166 of lowersection 162 by an angle a. Alternatively, the upper section 161 and thelower section 162 may formed with a fixed angular offset. The tool face168 of bent sub 169 is defined herein, see FIG. 4A, to indicate wherethe upper surfaces of upper section 161 and lower section 162 intersectwith the plane 191 that contains upper section axis 163 and lowersection axis 166, at the inward bend point on the outer surfaces. Thebit 150 will have a tendency to drill in the direction of the bent subtool face, for example, upwardly in the direction normal to the vertexand in plane 191, as shown in the 2-dimensional illustrations of FIGS.4A and 4B. It will be appreciated by one skilled in the art, that, inoperation, the bent sub toolface 168 may be positioned at any rotationalangle in the borehole to allow 3-dimensional steering of BHA 159.

In one embodiment, the toolface 168 of bent sub 169 and attacheddrilling motor housing 181 may be referenced relative to the high sideof the wellbore by using a directional sensor package 195 that may belocated in sensor sub 196. Measurements from the directional sensors maybe used to determine the toolface 168 of bent sub 169 with respect togravity and/or magnetic north using techniques known in the art. If agyroscope is used, a gyro north may be referenced. The wellbore highside is commonly referenced to the gravity high side for wellboreinclinations greater than about 5°. For inclinations of about 5°, orless, magnetic north may referred to as the wellbore high side.Communications of measured data between sensor sub 196 and orienter 190may be achieved via use of an acoustic or electromagnetic telemetryshort hop technique, or by a slip ring 235 (see FIG. 3), or an inductivecoupler. These short hop telemetry techniques are well known in the art,as are slip rings and inductive couplers. Via such linkages, data fromthe directional sensor package 195 can be conveyed to the orienter 190in real time for real time control of the orienter clutch. Using thesemeasurements, the bent sub toolface may be maintained within apredetermined range by adjusting the clutch slippage such that the drillstring above orienter 190 may continuously rotate while the drillingmotor and bent sub toolface remain oriented substantially at the desiredtoolface to drill in the desired direction. Alternatively, directionalsensor package 195 and sensor sub 196 may be located at any suitableposition in the lower portion of BHA 159 that rotates with orientershaft 170. In one example, bent sub toolface readings may becommunicated to telemetry transmitter 133 and then transmitted tosurface control unit 140 for processing and display. Surface controlunit 140 may transmit changes in bent sub toolface to controller 240using suitable downlink techniques known in the art.

In another embodiment, also referring to FIG. 5, a BHA 259 comprises adifferent arrangement of downhole tools. In BHA 259, orienter 190 isattached to drill string 120. Attached to orienter shaft is MWD 158,motor 180 and bent sub 169. As before, bit 150 is attached to motorshaft 175. In this arrangement, MWD 158 rotates with the motor and bentsub. As such, the MWD directional sensor package 295 may be useddetermine the toolface of bent sub 169 in the wellbore. In one example,this data may be communicated to orienter 190 using any of the short hoptechniques described above. In another example, the data may betransmitted by hard wire along, or in, shaft 170 and transferred by slipring 235 to orienter controller 240 (see FIG. 3). Alternatively, aninductive coupler may be used to transfer information to orientercontroller 240. As described above, the relative motion of orientershaft 170 relative to orienter housing 210 allows BHA 159 to beconsidered to have an upper BHA section 156 that rotates with drillstring 120, and a lower BHA section 157 that rotates at the same speedas the drill string, at a different speed as the drill string 120, orthat is substantially non-rotating. In one example, if drill string 120is rotating slowly, orienter shaft 170 may rotate in an oppositedirection relative to drill string 120.

In one embodiment, orienter controller 240 comprises a processor 241 indata communication with a memory 242, and interface circuits 243.Processor 241 may be any processor suitable for downhole use. Memory 242may comprise RAM, ROM, EPROM EEPROM, flash memory, or any other suitablememory. In one example, orienter controller 240 may be programmed at thesurface with appropriate bent sub target toolface angles for drilling asection of the wellbore. In addition, commands from surface control unit140 may be transmitted downhole to adjust the target toolfaceorientation. In yet another example, a target directional well path maybe stored in controller 240. Downhole directional measurements may beused to determine an actual well path, and/or deviations from the targetpath. Programmed instructions stored in controller 240 may be used toadjust the toolface of bent sub 169 to adjust the wellbore trajectoryback along the target path. Alternatively, directional measurements maybe transmitted to the surface to determine any needed trajectorycorrections. New target toolface values may then be downlinked tocontroller 240 for execution.

In operation, in one example, the hydraulic pressure on piston 225 (seeFIG. 3) is increased such that the frictional force on clutch plates226, 227 is increased to the point that orienter housing 210 andorienter shaft 170 are locked and there is substantially no differentialrotation between orienter housing 210 and orienter shaft 170. Then bentsub 169 rotates at the substantially the same rate as drill string 120.This condition allows straight ahead drilling of wellbore 126. When itis desired to deviate the wellbore from a straight direction, a bent subtoolface may be calculated and positioned to cause the path to turn inthe desired direction by setting a new target toolface in orientercontroller 240. In one example, the pressure on piston 225 is reduced tothe point that orienter shaft 170 rotates relative to orienter housing210. Due to the reactive torque, bent sub 169 rotates opposite thedirection of bit rotation. The bent sub toolface is monitored bydirectional sensors 195 and/or 295. When the bent sub toolface is withinan acceptable range of the new target toolface, the pressure on piston225 is controllably increased to maintain the bent sub toolface withinthe allowable range. In one example, see FIG. 7, the allowable toolfacerange is about ±45° around the target toolface 710. As the drill stringRPM is changed, changes in bent sub toolface are sensed by theappropriate sensors and fed back to orienter controller 240. Orientercontroller 240 adjusts the pressure on piston 225 thereby adjusting thefrictional force on clutch plates 226 and 227 to provide a controllableslip between clutch plates 226 and 227 to maintain the bent sub toolfacewithin the acceptable range about the target toolface.

This operational method is shown in FIG. 6, where the clutch may belocked in logic box 610 such that the orienter shaft 170 and orienterhousing 210 both rotate at the same RPM as drill string 120 in logic box620. When a new direction is desired, a new target bent sub 169 toolfacemay be calculated and transmitted to orienter controller 240. Orientercontroller 240 receives the new target toolface in logic box 630. Theclutch is unlocked in logic box 640 and the bent sub is allowed torotate relative to the orienter housing 210. The bent sub rotation ismonitored in logic box 650. When the bent sub toolface is within theacceptable range, the clutch friction is increased in logic box 660. Thedrill string continues to rotate and the clutch friction is adjusted tomaintain the bent sub toolface within the acceptable target toolfacerange in logic box 670. This process may be repeated each time a newbent sub toolface is desired.

In yet another embodiment, see FIGS. 9 and 10, a bottom hole assembly959 comprises an orienter 990. Orienter 990 is similar to orienter 190as described with respect to FIGS. 2 and 3. Orienter 990 furthercomprises a directional sensor package 937 located in housing 210 oforienter 990. Housing 210 is a component of upper portion 956 of BHA 959and rotates with drill string 120. Since the sensor package 937 rotateswith the upper portion 956, a referencing sensor 940 provides relativerotational orientation between the upper section 956 and the lowersection 957. Referencing sensor 940 may comprise an inner ring 936attached to an end of shaft 170, and an outer ring 935 attached tohousing 210. The relative position of rings 935 and 936 may bedetermined using magnetic, inductive, and/or optical techniques known inthe art. Then, the actual orientation of housing 210, provided bydirectional sensor package 937, along with the relative position ofshaft 170 with respect to housing 210, provided by referencing sensor940, allows the actual orientation of lower assembly 957 to bedetermined.

In another embodiment, see FIG. 11, BHA 1059 comprises an orienter 990that is reversed in direction from that of FIG. 9. This orientationallows the directional sensor package 937 (see FIG. 10) to be referencedto the bent sub high side using a simple mechanical offset measurementat assembly. The directional sensor package may make direct measurementsof the high side orientation without requiring a separate referencingsensor.

In yet another embodiment, shown in FIGS. 12A and 12B, a steeringapparatus 1260 is disposed in BHA 1259. Steering apparatus 1260 maycomprise a steerable assembly 1269 comprising at least one controllablyextendable member 1291 disposed around the perimeter of steering subbody 1220. Controllably extendable member 1291 may be extended tocontact a wall of borehole 126 to cause bit 150 to drill in apredetermined direction. Actuators 1290 may extend and retract member1291 to any position between fully extended and fully retracted. In oneexample, a sensor 1250 may detect the position of extendable member 1291and transmit a signal representative thereof to steerable assemblycontroller 1240. Steerable assembly controller 1240 may be in datacommunication with a directional sensor package (not shown) in BHA 1259for determining a high side of steering apparatus 1260, as previouslydescribed. Steerable assembly controller 1240 may also be in datacommunication with a controller in orienter 990. The controller inorienter 990 may provide instructions for adjusting extendable members1291 to steerable assembly controller 1240 to change or maintain thepath of drill bit 150 along a predetermined path. In one example,actuators 1290 may be hydraulic actuators extending shafts 1292 toposition extendable members 1291 at appropriate positions. Hydraulicsource 1246 may supply hydraulic fluid to actuators 1292 under directionof steerable assembly controller 1240. Orienter 990 may be operated tomaintain the high side of steerable assembly 1269 within a predeterminedrange of a target high side while controlling the position of extendablemembers 1291.

Alternatively, in still another embodiment, see FIGS. 13A-13D, asteering apparatus 1360 comprises a steerable assembly 1369 that may beattached to any of the BHA's previously discussed. Steerable assembly1369 comprises a rotatable drilling shaft 1375, coupled to bit 150. Ahousing 1346 rotatably supports at least a length of drilling shaft1375. A deflection assembly 1320 is contained within housing 1346 forbending the drilling shaft 1375 between a first bearing 1315 and asecond bearing 1335. Deflection assembly 1320 comprises an outer ring1345 which is rotatably supported on a circular inner peripheral surfaceof housing 1346. Outer ring 1345 has a circular inner peripheral surfacethat is eccentric with respect to housing 1346. Deflection assembly 1320also comprises an inner ring 1350 which is rotatably supported on thecircular inner peripheral surface of outer ring 1345 and which has acircular inner peripheral surface that engages drilling shaft 1375, andwhich is eccentric with respect to the circular inner peripheral surfaceof outer ring 1345. By rotating the outer ring 1345 and the inner 1350relative to each other, and to housing 1346, shaft 1375 may be deflectedby a distance, e, such that bit 150 drills in a direction, 8, from theprevious borehole centerline. Outer ring 1345 and inner ring 1350 may becontrollably rotated relative to each other and to housing 1346 by anactuator 1325. In one example, actuator 1325 may comprise a motor drivengear drive for rotating the outer ring 1345 and the inner ring 1350relative to each other. Alternatively, actuator 1325 may comprise aclutch that engages a harmonic drive gear system coupling power fromdrilling shaft 1375 to rotate outer ring 1345 and inner ring 1350relative to each other and to housing 1346, thereby deflecting drillingshaft 1375 a predetermined amount. Controller 1326 may comprise aprocessor in data communications with the controller in any of theorienters described above. In one embodiment, the toolface of housing1346 may controlled using the orienters described above, and thedeflection of drilling shaft 1375 may be controlled with respect to thetoolface angle by the proper rotation of outer ring 1345 and inner ring1350 relative to the toolface of housing 1346.

Numerous variations and modifications will become apparent to thoseskilled in the art. It is intended that the following claims beinterpreted to embrace all such variations and modifications.

1. An apparatus for drilling a wellbore comprising: a rotatable drillstring; a bottom hole assembly coupled to the drill string, the bottomhole assembly comprising an upper section and a lower section, the lowersection comprising a drilling motor, and a steering apparatus having atoolface reference; a sensor disposed in the bottom hole assembly toprovide at least a measure of the toolface angle; a controllablyadjustable clutch coupling the upper section to the lower section; and acontroller in data communication with the sensor and with thecontrollably adjustable clutch, the controller adjusting the clutch tomaintain a steering assembly toolface angle in a predetermined rangeabout a target steering apparatus toolface angle while drilling.
 2. Theapparatus of claim 1 wherein the controller adjusts the clutch tomaintain the steering apparatus toolface angle in the predeterminedrange about the target steering apparatus toolface angle while the uppersection rotates with the drill string.
 3. The apparatus of claim 1wherein the controllably adjustable clutch comprises: a housing coupledto one of the upper section and the lower section; and a shaft rotatablysupported by the housing, the shaft coupled to the other of the uppersection and the lower section.
 4. The apparatus of claim 3 furthercomprising a housing clutch plate engaged with the housing and a shaftclutch plate engaged with the shaft wherein an adjustable frictionalforce between the housing clutch plate and the shaft clutch platecontrols the relative rotation between the upper section and the lowersection.
 5. The apparatus of claim 1 wherein the controller comprises aprocessor in data communications with a memory.
 6. The apparatus ofclaim 5, wherein the processor acts according to programmed instructionsstored in the memory to adjust a frictional force between a housingclutch plate and a shaft clutch plate based on a measurement of thetoolface to maintain a target bent sub toolface within a predeterminedtoolface range.
 7. The apparatus of claim 1 wherein the predeterminedrange is about ±45° around the target toolface.
 8. The apparatus ofclaim 1 wherein the sensor comprises a directional sensor packagecomprising at least one of: an inclinometer; a magnetometer; and agyroscope.
 9. The apparatus of claim 8 further comprising a referencesensor.
 10. The apparatus of claim 8 wherein the directional sensorpackage is located in the bent sub.
 11. The apparatus of claim 8 whereinthe directional sensor package is located in the housing.
 12. Theapparatus of claim 8 wherein the directional sensor package is disposedin a measurement while drilling tool disposed in the lower section. 13.The apparatus of claim 1 wherein the drill string comprises a wireddrill pipe section.
 14. The apparatus of claim 1 wherein the steeringapparatus is chosen from the group consisting of a bent sub and asteerable assembly.
 15. The apparatus of claim 14 wherein the steerableassembly comprises at least one extendable member to cause the lowersection to drill in a predetermined direction.
 16. The apparatus ofclaim 14 wherein the steerable assembly comprises a controllablydeflectable drilling shaft to cause the lower section to drill in apredetermined direction.
 17. A method for drilling a wellborecomprising: extending a rotatable drill string in the wellbore, thedrill string having a bottom hole assembly coupled to a bottom endthereof; coupling a lower section of the bottom hole assembly comprisinga steering apparatus to an upper section of the bottom hole assemblywith a controllably adjustable clutch; detecting a steering apparatustoolface angle; controllably adjusting the clutch to maintain thesteering apparatus toolface angle within a predetermined range about atarget steering apparatus toolface angle while rotating the uppersection with the drill string.
 18. The method of claim 17 furthercomprising transmitting the detected steering apparatus toolface angleto a surface control unit.
 19. The method of claim 18 further comprisingdownlinking an updated steering apparatus target toolface from thesurface to a downhole controller.
 20. The method of claim 17 furthercomprising storing a directional model in a downhole memory in datacommunication with a processor in a downhole controller.
 21. The methodof claim 18 further comprising calculating a new steering apparatustarget toolface using the directional model stored in the downholememory.
 22. The method of claim 17 wherein detecting a steeringapparatus toolface angle comprises disposing a directional sensorpackage in one of the upper section and the lower section of the bottomhole assembly.
 23. The method of claim 17 further comprisingtransmitting a detected steering apparatus toolface to a controller inone of the upper section and the lower section.